In all the applications above, there is a need of creating exploded views of assemblies or sub-assemblies, to illustrate a nomenclature for instance. An example of such an exploded view is provided in FIG. 1. As it can be seen on the figure, dashed polylines AL, called “assembly lines”, are used to indicate the original location of the parts and the way they have been removed from there (assembly paths). In order to allow a good understanding of the exploded view, in most cases (but not always) the segments of the polyline are mutually perpendicular and usually oriented parallel or perpendicular to a “world axis” of the view.
A first possibility for designing assembly lines consists in drawing the polylines freely in the plane defined by the current point of view. Otherwise stated, the polyline is drawn in two dimensions (2D), and automatically integrated into the three-dimensional representation of the scene. The angle between consecutive segments of the polyline may not take accurate values (e.g. 45° or 90°); most importantly, as the polyline is not properly situated in three-dimensions, they are lost if the user changes the point of view. This is illustrated on FIG. 2: the left part of the figure shows a polyline drawn in two-dimensions, which gives the illusion of being constituted of mutually perpendicular segments in 2D space; the illusion is completely lost if the scene is rotated in three dimensions.
Another approach, illustrated on FIG. 3, uses planar grids to properly draw the segments of the polyline. A drawback of this technique is that it is quite cumbersome to create and position a grid before drawing each segment. Also, drawing a complex three-dimensional polyline requires the simultaneous display of several grids, which may be confusing for the user and demanding (essentially in terms of memory space) for the Computer Aided Design/illustration authoring system.
Yet another approach is to use software for automatically creating a path (i.e. a polyline) between the original position of a part to a new one. This approach is not flexible enough, as the user has no control on the position of the “breakings” of the line. The result is often unsatisfactory for a human end user, as depicted on FIG. 4.